Marginal impaction associated with posterior malleolar fracture in rotational ankle injury

•Posterior malleolar fractures (PMFs) need to be precisely investigated preoperatively using computed tomography (CT) scan.•We introduced the circle drawing method to evaluate marginal impaction associated with PMFs on CT images.•The marginal impaction associated with PMFs is often encountered in rotational ankle fractures.•Our findings and the circle drawing method could facilitate improvements in the management of PMFs. This study aimed to introduce the circle drawing method for the evaluation of marginal impaction associated with posterior malleolar fractures (PMFs) on computed tomography (CT) images and evaluate the marginal impaction based on PMF patterns (Bartonícek classification) in rotational ankle fractures. From January 2014 to December 2019, A total of 299 patients were reviewed retrospectively. The circle drawing method consists of matching the articular surface of the intact tibial plafond with a best-fitted circle on the sagittal CT image. The mismatch gap between the circle and the articular surface indicates the presence of marginal impaction. To validate the circle drawing method, we assessed CT images of 60 ankles without fracture at the medial, central, and lateral parts using the new method. Based on the review of the preoperative CT scan, 153 out of 299 patients (51.2%) were identified as having PMFs. Among them, 57 patients (37.3%) had marginal impaction associated with PMFs. For small posterolateral type 2 fractures, 58% of patients (47/81) had marginal impaction, and its incidence was most frequent among all types (P